1. Field
The present invention relates to a semiconductor substrate, a semiconductor device, and a method of fabricating the same. More particularly, the present invention relates to a process of lifting-off a gallium nitride layer from a sapphire substrate, a semiconductor substrate and a semiconductor device fabricated through the lift-off process, and a method of fabricating the same.
2. Discussion of the Background
Gallium nitride (GaN) based light emitting diodes (LEDs) have been used in a wide range of applications including signals, a backlight unit of a liquid crystal panel, etc.
Since fabrication of a gallium nitride (GaN) substrate is very difficult and requires high manufacturing costs, semiconductor devices such as LEDs or laser diodes are generally fabricated by growing a GaN layer on a heterogeneous substrate such as sapphire. One example of crystal growth of GaN is disclosed in “Polycrystalline GaN for light emitter and field electron emitter applications” (S. Hasegawa, S. Nishida, T. Yamashita, H. Asahi, Thin Solid Films 487 (2005), pp. 260-267). In this document, a GaN crystal is grown on a quartz substrate, a high melting-point metal substrate of W, Mo, Ta and Nb, and a Si substrate through plasma-assisted molecular beam epitaxy.
However, lattice mismatch and different coefficients of thermal expansion between the GaN layer and the substrate cause high dislocation density or increases defects, obstructing improvement of luminous efficiency of an LED. Although mechanical polishing or laser ablation is performed to strip a GaN bulk crystal into a GaN substrate, it is very difficult to obtain a GaN substrate with practical size and good reproducibility. In addition, since a sapphire substrate has lower thermal conductivity than the GaN substrate, the sapphire substrate deteriorates heat dissipation of a semiconductor device. Further, when a thin GaN layer is formed on the sapphire substrate, it is very difficult to lift-off the GaN layer from the sapphire substrate.